1. Field of the Invention
The present invention relates to a moisture-sensitive element and a humidity detector using the same.
2. Prior Art
A conventional moisture-sensitive element uses an organic polymer such as celluloseacetatebutyrate, celluloseacetatepropyonate, polyimide, or polyimideamide as a moisture-sensitive material and utilizes an electrical capacitance change in moisture-sensitive film formed by this moisture-sensitive material to detect humidity (Japanese Patent Laid-Open No. 62-88951).
A moisture-sensitive element having the above arrangement, however, has a high hydrophilic nature and a large water sorption amount (water absorption ratio). Therefore, a large amount of water strongly bonded to the polymer remains. For this reason, when the element is used in a high-temperature, high-humidity atmosphere, e.g., at a temperature of 40.degree. C. and a humidity of 90% for a long period of time, its output value drifts. In addition, a moisture-sensitivity characteristic difference (hysteresis) between moisture absorption and desorption processes is decreased at a low-temperature side and increased at a high-temperature side, thereby delaying sensor response. Furthermore, an output value of the element drifts due to moisture condensation or water dipping.
In a conventional moisture-sensitive element, a moisture-sensitive film is sandwiched between lower and upper electrodes, and an electrical signal is extracted from external lead wires directly connected to terminals of the electrodes.
In the moisture-sensitive element having the above arrangement, however, the external lead wires are electrically and mechanically connected to the electrode terminals by solder. Therefore, a mechanical stress or vibration is externally applied to the lead wires, or degradation occurs due to a repetitive moisture absorption/dry cycle or a temperature cycle, thereby causing a contact failure. A moisture-sensitive element using conductive rubber in place of an external lead wire to be connected to electrode terminals is available. In this moisture-sensitive element, however, fatigue or permanent deformation of the conductive rubber or a vibration leads to a contact failure. A moisture-sensitive element using a conductive adhesive in place of the solder for connecting the electrode terminals and the external lead wires is also available. In this moisture-sensitive element, however, when a mechanical stress or vibration is externally applied to the external lead wires, the force directly acts on the conductive adhesive portion having low strength. Therefore, film peeling or a contact failure tends to occur upon handling or actual use, resulting in poor handling properties and reliability.
Such a moisture-sensitive element is incorporated in a humidity detector as disclosed in, e.g., Japanese Utility Model Laid-Open No. 61-69153.
In such a humidity detector, however, although a porous filter consisting of a repellent material is mounted in an opening portion of a main body case to prevent attachment of water, dust, tobacco smoke, salts or the like to the surface of the moisture-sensitive element, a sealing property between the filter and the main body case is unsatisfactory. Therefore, when the detector is dipped and used in water, water reaches the moisture-sensitive element to cause inconvenience. For this reason, after the humidity detector is used for a long time period, it cannot be washed with water to remove foreign matters adhered on the porous filter.